In today's unsafe and increasingly wired world cryptology plays a vital role in protecting communication channels, databases, and software from unwanted intruders. This revised and extended third edition of the classic reference work on cryptology now contains many new technical and biographical details. The first part treats secret codes and their uses - cryptography. The second part deals with the process of covertly decrypting a secret code - cryptanalysis, where particular advice on assessing methods is given. The book presupposes only elementary mathematical knowledge. Spiced with a wealth of exciting, amusing, and sometimes personal stories from the history of cryptology, it will also interest general readers.